Uniparental disomy of chromosome 16 unmasks recessive mutations of FA2H/SPG35 in 4 families.

Soehn AS¹, Rattay TW¹, Beck-Wödl S¹, Schäferhoff K¹, Monk D¹, Döbler-Neumann M¹, Hörtnagel K¹, Schlüter A¹, Ruiz M¹, Pujol A¹, Züchner S¹, Riess O¹, Schüle R¹, Bauer P², Schöls L¹.

Objective:
Identifying an intriguing mechanism for unmasking recessive hereditary spastic paraplegias.

Method:
Herein, we describe 4 novel homozygous FA2H mutations in 4 nonconsanguineous families detected by whole-exome sequencing or a targeted gene panel analysis providing high coverage of all known hereditary spastic paraplegia genes.

Results:
Segregation analysis revealed in all cases only one parent as a heterozygous mutation carrier whereas the other parent did not carry FA2H mutations. A macro deletion within FA2H, which could have caused a hemizygous genotype, was excluded by multiplex ligation-dependent probe amplification in all cases. Finally, a microsatellite array revealed uniparental disomy (UPD) in all 4 families leading to homozygous FA2H mutations. UPD was confirmed by microarray analyses and methylation profiling.

Conclusion:
UPD has rarely been described as causative mechanism in neurodegenerative diseases. Of note, we identified this mode of inheritance in 4 families with the rare diagnosis of spastic paraplegia type 35 (SPG35). Since UPD seems to be a relevant factor in SPG35 and probably additional autosomal recessive diseases, we recommend segregation analysis especially in nonconsanguineous homozygous index cases to unravel UPD as mutational mechanism. This finding may bear major repercussion for genetic counseling, given the markedly reduced risk of recurrence for affected families.